It is in the city and state ordinances, all easily available online. Don't waste your time on city ordinances, every incorporated city within at least 2 hours of Dallas has banned new billboards in entirety. You'll need to get the state requirements and start in county land.

I just did a Google search on "texas statutes billboards" and "texas billboard regulations" and the first page were all kinds of links to the legal statutes and even a legal pdf that summarized the rules at the state level. Give those searches a try and you'll have plenty to read.

I'm just starting out and have spent more time getting a fuller understanding of the rules, potential rent vs traffic count, what cities still allow them, etc. hoping to make the search more efficient later. It is interesting how secrative this industry is due, I imagine, to the scarcity of legal locations. You will find very little useful hard data out there and even fewer people in the industry being helpful. The lack of data is quite distressing to me as I am an engineer in my day job and my inclination isto get the numbers. It can only mean there must be money to be had. Doesn't make it any easier to do however.

For what little I've done in searching, I'll say this: don't expect to cruise down I35 and find an awesome location just sitting there. Frank and his buddies locked up all the low hanging fruit 30 years ago, and the rules have only gone up, way up, in obstacles.